A modified attapulgite clay and its preparation method

Abstract

This invention relates to the field of cementitious materials technology, specifically to a modified attapulgite clay and its preparation method. The method involves a synergistic process of acid washing, gradient calcium ion preloading, stepwise introduction of anionic polyelectrolytes of different molecular weights as templates, carbon dioxide-induced mineralization followed by cationic polyelectrolyte capture and secondary carbonization to modify the surface of natural attapulgite clay. When the resulting powder is applied to cement-based materials, it effectively reduces the water demand of the system, improves the fluidity of freshly mixed slurry, and significantly enhances early strength due to the dense carbonate functional layer formed on the surface. Simultaneously, it inhibits the drying shrinkage of the hardened body, solving the problems of poor dispersibility, insufficient early strength contribution, and difficulty in synergistically improving volume stability in existing technologies.

Description

Technical Field

[0001] This invention relates to the field of cementitious materials technology, and in particular to a modified attapulgite clay and its preparation method. Background Technology

[0002] Attapulgite clay is a natural silicate mineral with a unique one-dimensional nanorod crystal structure. Due to its large specific surface area and ion exchange capacity, it has been widely used in adsorption, catalysis, and other fields. In recent years, its application as a functional mineral admixture in cement-based or solid waste-based cementitious material systems to improve mechanical properties or achieve low-carbonization has become one of the research hotspots in the field of building materials.

[0003] However, when unmodified natural attapulgite clay is directly incorporated into a cementitious system, its inherent technical defects severely restrict its full performance. First, the rod-shaped crystals in natural attapulgite clay often aggregate in bundles, resulting in an actual effective specific surface area far lower than the theoretical value. Furthermore, this high specific surface area significantly increases the water demand of the cementitious system, negatively impacting the workability of the freshly mixed paste. Second, although its surface possesses a certain degree of activity, the density and distribution of active sites are not ideal, making it difficult to efficiently induce heterogeneous nucleation of hydration products in the early stages of cement hydration, thus limiting its contribution to early strength.

[0004] More importantly, in low-carbon cementitious material formulations that pursue high solid waste content or low clinker content, the system's alkalinity and ionic composition are complex, placing higher demands on the interfacial and volumetric stability of mineral admixtures. Current technologies for modifying attapulgite clay often focus on single methods such as acid activation, ion exchange, or organic encapsulation. These methods often only improve one aspect of its properties, such as increasing adsorption or dispersibility in a specific system, but they cannot systematically address the multiple challenges it faces in complex cementation environments. For example, while simple acid treatment can partially dissociate crystal bundles, it may introduce interfering ions, affecting subsequent reactions; while single organic modification can improve dispersion, it may delay hydration or introduce harmful pores, leading to insufficient strength development and increased drying shrinkage in the later stages of hardening.

[0005] Therefore, existing technologies lack a modification method that can fundamentally reconstruct the surface properties of attapulgite clay, enabling it to simultaneously achieve low water demand, high early strength contribution, and excellent volume stability in cementitious materials. This has become a technical bottleneck restricting its large-scale application in high-end low-carbon building materials. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a modified attapulgite clay and its preparation method, so as to solve the technical problems of low early strength and poor volume stability caused by poor dispersibility, high water demand, insufficient early nucleation efficiency and weak interfacial anchoring of existing attapulgite clay in cementitious materials, which are difficult to improve in a coordinated manner.

[0007] To achieve the above objectives, the present invention provides a method for preparing modified attapulgite clay, comprising the following steps: (1) Acid washing was performed on the attapulgite clay, and then it was washed until neutral to obtain neutral sediment. (2) Calcium nitrate solution A and calcium nitrate solution B are added sequentially to the neutral sediment for ion exchange to achieve calcium ion preloading, and the sediment is washed to obtain calcium ion preloaded sediment, wherein the calcium nitrate concentration in calcium nitrate solution A is higher than the calcium nitrate concentration in calcium nitrate solution B. (3) First, add poly(4-styrene sulfonate) solution C to the calcium ion preloaded sediment for reaction, and then add poly(4-styrene sulfonate) solution D for reaction to obtain a composite precursor slurry, wherein the weight-average molecular weight of poly(4-styrene sulfonate) in solution C is less than the weight-average molecular weight of poly(4-styrene sulfonate) in solution D. (4) Add calcium nitrate solution B and sodium bicarbonate to the composite precursor slurry, add sodium hydroxide alkaline solution to adjust the pH of the system to 9.8-10.2, and introduce carbon dioxide gas under stirring to reduce the pH of the system from 9.8-10.2 to 8.8-9.2 and mature it; after maturation, add anhydrous sodium carbonate to raise the pH of the system to 11.0-11.3; then add poly(diallyldimethylammonium chloride) aqueous solution to allow it to charge-pair with poly(4-styrene sulfonate) and form a precipitable polyelectrolyte complex, and wash the precipitate; (5) The washed sediment was dispersed in a saturated calcium hydroxide solution and carbon dioxide gas was introduced for secondary carbonization to reduce the pH of the system to 10.6-11.5. The product after secondary carbonization was vacuum dried, crushed and sieved to obtain modified attapulgite clay.

[0008] Preferably, in step (1), the pickling process is continued to be stirred for 20-40 minutes at 35-45°C.

[0009] Preferably, in step (1), after acid washing, the supernatant is washed until the pH reaches 6-7 and the conductivity is ≤1200μS / cm.

[0010] Preferably, in step (1), the average particle size of the attapulgite clay is 40-50 μm.

[0011] Preferably, based on 500 parts by mass of attapulgite clay, the preparation of calcium nitrate solution A is as follows: heat deionized water to 35-45°C, add 500-700 parts by mass of calcium nitrate tetrahydrate to dissolve, and then add deionized water to make up to 5000 parts by mass; the preparation of calcium nitrate solution B is as follows: dissolve 80-150 parts by mass of calcium nitrate tetrahydrate in deionized water and then add deionized water to make up to 5000 parts by mass.

[0012] Preferably, in step (2), the ion exchange is carried out at 45-55°C, wherein the ion exchange reaction time of calcium nitrate solution A is 15-30 min and the ion exchange reaction time of calcium nitrate solution B is 90-180 min.

[0013] Preferably, based on 500 parts by mass of attapulgite clay, the poly(4-styrene sulfonate) solution C is prepared by: heating deionized water to 35-45°C, dispersing and adding 4-10 parts by mass of poly(4-styrene sulfonate) and stirring until completely dissolved, then adding deionized water to bring the total to 4000 parts by mass, wherein the weight-average molecular weight of the poly(4-styrene sulfonate) is 60000-80000; the poly(4-styrene sulfonate) solution D is prepared by: heating deionized water to 35-45°C, sprinkling in 0.5-2 parts by mass of poly(4-styrene sulfonate) and stirring for 2 hours, then adding deionized water to bring the total to 1000 parts by mass, wherein the weight-average molecular weight of the poly(4-styrene sulfonate) is 900000-1100000.

[0014] Preferably, in step (3), after adding poly(4-styrene sulfonate) solution C and reacting for 10-25 minutes, poly(4-styrene sulfonate) solution D is added at a uniform rate within 8-15 minutes, and then stirring is continued for 10-15 minutes.

[0015] Preferably, based on 500 parts by mass of attapulgite clay, in step (4), 800-1500 parts by mass of calcium nitrate solution B are added first and stirred for 10 minutes, followed by 80-140 parts by mass of sodium bicarbonate and stirred for 10 minutes to completely dissolve it, and the system is kept at a constant temperature of 28-33°C.

[0016] Preferably, in step (4), after stopping the introduction of carbon dioxide gas, stirring is continued for 15-30 minutes for maturation. After maturation, anhydrous sodium carbonate is added and stirred for 10 minutes to raise the pH of the system to 11.0-11.3.

[0017] Preferably, based on 500 parts by weight of attapulgite clay, in step (4), the concentration of the poly(diallyldimethylammonium chloride) aqueous solution is 20 wt%, the weight-average molecular weight is 250,000-350,000, and the amount added is 20-60 parts by weight.

[0018] Furthermore, the present invention also provides a modified attapulgite clay, obtained by the above-described method for preparing modified attapulgite clay.

[0019] The beneficial effects of this invention are: First, gradient ion exchange and preloading were performed using calcium salt solutions of specific concentrations, allowing calcium ions to fully occupy the outer surface and internal pores of attapulgite clay, significantly increasing the density of active sites available for mineralization reactions. This laid a crucial ionic foundation for the subsequent in-situ construction of a uniform and continuous inorganic functional layer on its surface.

[0020] Secondly, an innovative stepwise introduction strategy using anionic polyelectrolytes of different molecular weights was employed. First, lower molecular weight polyelectrolytes penetrated into the internal pores and intergranular spaces of the particles, coordinating with pre-loaded calcium ions to construct an internal nucleation template. Subsequently, higher molecular weight polyelectrolytes formed a coating layer on the outer surface of the particles. This internal-to-external spatial gradient template design guides the preferential growth of inorganic minerals at the interface and within the pores, effectively inhibiting unwanted free precipitation and particle re-agglomeration in the bulk phase. This reduces the overall specific surface area and water requirement of the material while improving its dispersion stability in the cementation system.

[0021] Third, during the mineralization stage, preliminary calcium carbonate crystal nuclei are formed by inducing the formation of carbon dioxide gas, followed by pH adjustment and densification using anhydrous sodium carbonate. This two-stage mineralization sequence control promotes the formation of a fine, uniform, and structurally robust carbonate functional layer. This method avoids the coarse crystals that are easily produced by conventional co-precipitation methods, making the formed functional layer more conducive to playing a role in micro-aggregate filling and interface reinforcement in cementitious materials.

[0022] Finally, by introducing cationic polyelectrolytes for post-capture and washing, the anionic polyelectrolytes serving as templates can be effectively removed, reducing the potential adverse effects of organic residues on the hydration process of the cementitious material. The subsequent secondary carbonization treatment in a saturated calcium hydroxide environment can repair and strengthen the formed functional layer, further enhancing its chemical stability and physical anchoring ability in the high-alkali environment of the cementitious material.

[0023] In summary, the modification scheme of this invention forms a complete and synergistic modification path, from surface purification, ion preloading, spatial template design, time-controlled mineralization to post-treatment strengthening. The resulting modified attapulgite clay, when incorporated with cement or other cementitious materials, can significantly reduce the standard consistency water requirement, improve the fluidity of freshly mixed slurry, and, due to the excellent nucleation sites and micro-filling effect provided by the firmly anchored carbonate functional layer on the surface, simultaneously promote the early strength development and long-term volume stability of the cementitious material, and inhibit drying shrinkage, thus perfectly meeting the development needs of low-carbon, high-performance cementitious materials. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0025] Example 1: Step S1: Place 1800g of deionized water in a container, add 200g of 37wt% hydrochloric acid while stirring to obtain 2000g of diluted hydrochloric acid solution; place 800g of deionized water in a container, add 200g of sodium hydroxide and cool to 25℃ under external cold water bath conditions to obtain 1000g of sodium hydroxide alkaline solution; heat 4410g of deionized water to 40℃ and add 590g of calcium nitrate tetrahydrate to dissolve to obtain 5000g of calcium nitrate solution A; add 118g of calcium nitrate tetrahydrate to 4882g of deionized water to dissolve to obtain 5000g of calcium nitrate solution B; add 3994g of deionized water... After heating water to 40°C, 6g of sodium poly(4-styrene sulfonate) (weight average molecular weight approximately 70,000) was dispersed and added, and stirred until completely dissolved to obtain 4000g of sodium poly(4-styrene sulfonate) solution C; after heating 999g of deionized water to 40°C, 1g of sodium poly(4-styrene sulfonate) (weight average molecular weight approximately 1,000,000) was slowly sprinkled in and stirred for 2 hours to obtain 1000g of sodium poly(4-styrene sulfonate) solution D; after adding 200g of calcium hydroxide to 10000g of deionized water, stirring for 30 minutes, and letting stand for 2 hours, 8000g of the clear supernatant was taken as saturated calcium hydroxide clear solution E; Step S2: Add 7500g of deionized water to the stirred tank, turn on the mechanical stirrer at 600rpm, add 500g of attapulgite clay (average particle size 45μm), and continue stirring for 20min to fully wet and initially disperse it; then add hydrochloric acid dilution while stirring to adjust the pH of the slurry to 3, and continue stirring at 40℃ for 30min; after standing for 20min, remove the supernatant, add 6000g of deionized water to the sediment and stir for 10min, repeat the standing for 20min and remove the supernatant; add another 6000g of deionized water to the sediment and stir for 10min, stand for 20min and remove the supernatant, repeat until the pH of the supernatant reaches 6-7 and the conductivity is ≤1000μS / cm, to obtain neutral sediment; Step S3: Add 5000g of calcium nitrate solution A to the neutral precipitate obtained in step S2, stir mechanically at 500rpm for 20min at 50℃, let stand for 20min and remove the supernatant; then add 5000g of calcium nitrate solution B, stir mechanically at 500rpm for 120min at 50℃, let stand for 20min and remove the supernatant; then add 4000g of deionized water, stir for 10min and let stand and remove the supernatant. Repeat twice to obtain calcium ion preloaded precipitate. Step S4: Add 4000g of poly(4-styrene sulfonate) solution C to the calcium ion preloaded sediment obtained in step S3 and mechanically stir at 600rpm for 15min at 25℃ to allow the lower molecular weight poly(4-styrene sulfonate) to preferentially enter the channels and intergranular gaps and electrostatically coordinate with the anchored calcium ions; then, under continuous stirring, add 1000g of poly(4-styrene sulfonate) solution D at a uniform rate over 10min, and continue stirring for another 10min to form a high molecular weight poly(4-styrene sulfonate) outer coating mainly distributed on the outer surface, to obtain the composite precursor slurry; Step S5: Add 1000g of calcium nitrate solution B to the composite precursor slurry obtained in step S4 and stir for 10 min. Then add 100g of sodium bicarbonate and stir for 10 min until completely dissolved. Next, add sodium hydroxide solution while stirring at 600 rpm to adjust the pH of the system to 10. Maintain the temperature at 30℃ and introduce carbon dioxide gas while stirring at 600 rpm to lower the pH of the system from 10 to 9. After stopping the gas introduction, continue stirring for 20 min for maturation. After maturation, add anhydrous sodium carbonate and stir rapidly at 800 rpm for 10 min to raise the pH of the system to 11. Then, reduce the stirring speed back to 600 rpm and continue stirring for 10 min. Finally, add 40g of poly(diallyldimethylammonium chloride) aqueous solution (concentration 20wt%, weight average molecular weight 3000) at a uniform rate over 5 min. 00), continue stirring for 15 min to allow it to undergo charge pairing with poly(4-styrene sulfonate) and form a precipitable polyelectrolyte complex; then add 6000 g of deionized water to the system, stir for 5 min, let stand for 20 min, and remove the supernatant. Repeat the washing process of adding 6000 g of deionized water and letting stand to remove the supernatant 3 times; add the washed precipitate to 8000 g of saturated calcium hydroxide clarified solution E and stir at 25 °C at 600 rpm for 10 min to disperse. Then, introduce carbon dioxide gas to lower the pH of the system to 11. After stopping the gas, continue stirring for 15 min, add 2000 g of deionized water, stir for 5 min, let stand, and remove the supernatant once more; dry the obtained product under vacuum at 60 °C for 12 h to constant weight, then pulverize and pass through a 200 mesh sieve to obtain modified attapulgite clay.

[0026] Example 2: Based on Example 1, in step S1: calcium nitrate solution A was obtained by heating 4300g of deionized water to 40°C and then adding 700g of calcium nitrate tetrahydrate to dissolve it, yielding 5000g; calcium nitrate solution B was obtained by adding 150g of calcium nitrate tetrahydrate to 4850g of deionized water to dissolve it, yielding 5000g; poly(4-styrene sulfonate) solution C was obtained by heating 3992g of deionized water to 40°C and then dispersing 8g of poly(4-styrene sulfonate) (weight average molecular weight approximately 70000) and stirring until completely dissolved, yielding 4000g; poly(4-styrene sulfonate) solution D was obtained by heating 998g of deionized water to 40°C and then slowly sprinkling in 2g of poly(4-styrene sulfonate) (weight average molecular weight approximately 1000000) and stirring for 2 hours, yielding 1000g. In step S2, the slurry pH is adjusted to 2.6 and stirred for 40 min at 45℃. Washing is continued until the supernatant pH reaches 6-7 and the conductivity is ≤800 μS / cm. In step S3, the displacement reaction time of calcium nitrate solution A is 25 min, and the displacement reaction time of calcium nitrate solution B is 150 min. In step S4, poly(4-styrene sulfonate) solution C is added and reacted for 20 min. Subsequently, poly(4-styrene sulfonate) solution D is added uniformly over 10 min, and stirring continues for 15 min. In step S5, 1200g of calcium nitrate solution B was added first, followed by 120g of sodium bicarbonate. The mixture was kept at a constant temperature of 32°C, and carbon dioxide gas was introduced to lower the pH of the system from 10 to 8.8. The mixture was allowed to mature for 25 minutes. After maturation, anhydrous sodium carbonate was added, and the mixture was stirred rapidly at 800 rpm for 10 minutes to raise the pH of the system to 11.2. Then, 50g of poly(diallyldimethylammonium chloride) aqueous solution was added uniformly over 5 minutes, and stirring continued for 20 minutes. During the second carbonization, carbon dioxide gas was introduced to lower the pH of the system to 10.8, and stirring continued for 20 minutes. Finally, the mixture was vacuum dried at 65°C for 10 hours to constant weight. The remaining conditions were the same as in Example 1, yielding modified attapulgite clay.

[0027] Example 3: Based on Example 1, in step S1: calcium nitrate solution A was prepared by heating 4500g of deionized water to 40°C and then adding 500g of calcium nitrate tetrahydrate to dissolve it, yielding 5000g; calcium nitrate solution B was prepared by heating 4900g of deionized water and then adding 100g of calcium nitrate tetrahydrate to dissolve it, yielding 5000g; poly(4-styrene sulfonate) solution C was prepared by heating 3995g of deionized water to 40°C and then dispersing 5g of poly(4-styrene sulfonate) (weight average molecular weight approximately 70,000) and stirring until completely dissolved, yielding 4000g; poly(4-styrene sulfonate) solution D was prepared by heating 999.2g of deionized water to 40°C and then slowly sprinkling 0.8g of poly(4-styrene sulfonate) (weight average molecular weight approximately 1,000,000) and stirring for 2 hours, yielding 1000g. In step S2, the pH of the slurry was adjusted to 3.2 and stirring was continued at 40°C for 25 minutes. In step S3, the displacement reaction time of calcium nitrate solution A was 15 min, and the displacement reaction time of calcium nitrate solution B was 100 min. In step S4, poly(4-styrene sulfonate) solution D was added uniformly over 15 min with continuous stirring. In step S5, 900 g of calcium nitrate solution B was added first, followed by 90 g of sodium bicarbonate. Carbon dioxide gas was introduced to lower the pH of the system from 10 to 9.2; the system was allowed to mature for 20 min; after maturation, anhydrous sodium carbonate was added and the system was stirred rapidly at 800 rpm for 10 min to raise the pH of the system to 11.0; then, 30 g of poly(diallyldimethylammonium chloride) aqueous solution was added uniformly over 5 min and stirring was continued for 15 min; the washing process was repeated twice; during the second carbonization, carbon dioxide gas was introduced to lower the pH of the system to 11.2 and stirring was continued for 15 min; finally, the system was vacuum dried at 55°C for 14 h to constant weight. The remaining conditions were the same as in Example 1, and modified attapulgite clay was obtained.

[0028] Example 4: Based on Example 1, in step S1: calcium nitrate solution A was prepared by heating 4350g of deionized water to 40°C and then adding 650g of calcium nitrate tetrahydrate to dissolve it, resulting in 5000g; calcium nitrate solution B was prepared by adding 80g of calcium nitrate tetrahydrate to 4920g of deionized water to dissolve it, resulting in 5000g; poly(4-styrene sulfonate) solution C was prepared by heating 3990g of deionized water to 40°C and then dispersing 10g of poly(4-styrene sulfonate) (weight average molecular weight approximately 70000) and stirring until completely dissolved, resulting in 4000g; poly(4-styrene sulfonate) solution D was prepared by heating 998.5g of deionized water to 40°C and then slowly sprinkling in 1.5g of poly(4-styrene sulfonate) (weight average molecular weight approximately 1000000) and stirring for 2 hours, resulting in 1000g. In step S2, the slurry pH is adjusted to 2.8 and stirred for 35 min at 42℃. The mixture is then washed until the supernatant pH reaches 6-7 and the conductivity is ≤900 μS / cm. In step S3, the calcium nitrate solution B displacement reaction takes 180 min. In step S4, poly(4-styrene sulfonate) solution C is added and reacted for 25 min. Subsequently, poly(4-styrene sulfonate) solution D is added uniformly over 8 min, followed by stirring for another 12 min. In step S5, 1500g of calcium nitrate solution B was added first, followed by 140g of sodium bicarbonate. The mixture was kept at a constant temperature of 28°C, and carbon dioxide gas was introduced to lower the pH of the system from 10 to 8.9. The mixture was allowed to mature for 30 minutes. After maturation, anhydrous sodium carbonate was added, and the mixture was stirred rapidly at 800 rpm for 10 minutes to raise the pH of the system to 11.3. Then, 60g of poly(diallyldimethylammonium chloride) aqueous solution was added uniformly over 5 minutes, and stirring was continued for 20 minutes. The washing process was repeated four times. During the second carbonization, carbon dioxide gas was introduced to lower the pH of the system to 10.6, and stirring was continued for 20 minutes. Finally, the mixture was vacuum dried at 70°C for 8 hours to constant weight. The remaining conditions were the same as in Example 1, yielding modified attapulgite clay.

[0029] Example 5: Based on Example 1, in step S1: calcium nitrate solution A was prepared by heating 4440g of deionized water to 40°C and then adding 560g of calcium nitrate tetrahydrate to dissolve it, resulting in 5000g; calcium nitrate solution B was prepared by adding 130g of calcium nitrate tetrahydrate to 4870g of deionized water to dissolve it, resulting in 5000g; poly(4-styrene sulfonate) solution C was prepared by heating 3996g of deionized water to 40°C and then dispersing 4g of poly(4-styrene sulfonate) (weight average molecular weight approximately 70000) and stirring until completely dissolved, resulting in 4000g; poly(4-styrene sulfonate) solution D was prepared by heating 999.5g of deionized water to 40°C and then slowly sprinkling in 0.5g of poly(4-styrene sulfonate) (weight average molecular weight approximately 1000000) and stirring for 2 hours, resulting in 1000g. In step S2, the slurry pH is adjusted to 3.5 and stirred for 20 minutes at 35°C. The slurry is then washed until the supernatant pH reaches 6-7 and the conductivity is ≤1200 μS / cm. In step S3, the calcium nitrate solution A displacement reaction time is 30 minutes, and the calcium nitrate solution B displacement reaction time is 90 minutes. In step S4, poly(4-styrene sulfonate) solution C is added and reacted for 10 minutes. In step S5, 800g of calcium nitrate solution B was added first, followed by 80g of sodium bicarbonate. The mixture was kept at a constant temperature of 33°C, and carbon dioxide gas was introduced to lower the pH of the system from 10 to 9.0. The mixture was allowed to mature for 15 minutes. After maturation, anhydrous sodium carbonate was added, and the mixture was rapidly stirred at 800 rpm for 10 minutes to raise the pH of the system to 11.0. Then, 20g of poly(diallyldimethylammonium chloride) aqueous solution was added uniformly over 5 minutes, and stirring continued for 15 minutes. During the second carbonization, carbon dioxide gas was introduced to lower the pH of the system to 11.5, and stirring continued for 15 minutes. Finally, the mixture was vacuum dried at 50°C for 16 hours to constant weight. The remaining conditions were the same as in Example 1, yielding modified attapulgite clay.

[0030] Comparative Example 1: The difference from Example 1 is that in step S3, only 5000g of calcium nitrate solution A is added, and the mixture is mechanically stirred at 500rpm for 140min at 50°C. After standing for 20min, the supernatant is removed, and calcium nitrate solution B is not added for long-term displacement. The other conditions are the same as in Example 1.

[0031] Comparative Example 2: The difference from Example 1 is that in step S4, 4000g of poly(4-styrene sulfonate) solution C and 1000g of poly(4-styrene sulfonate) solution D are mixed evenly and then added to the calcium ion preloaded sediment at one time. The mixture is then mechanically stirred at 600 rpm for 35 min at 25°C. The process is no longer carried out in the order of adding solution C and reacting for 15 min, then adding solution D at a uniform speed within 10 min and continuing to stir for 10 min. The other conditions are the same as in Example 1.

[0032] Comparative Example 3: The difference from Example 1 is that in step S4, instead of adding poly(4-styrene sulfonate) solution C, 5000g of poly(4-styrene sulfonate) solution D is added at once, and the mixture is mechanically stirred at 600rpm for 35min at 25°C; the other conditions are the same as in Example 1.

[0033] Comparative Example 4: The difference from Example 1 is that in step S4, instead of adding poly(4-styrene sulfonate) solution D, 5000g of poly(4-styrene sulfonate) solution C is added at once, and the mixture is mechanically stirred at 600rpm for 35min at 25°C; the other conditions are the same as in Example 1.

[0034] Comparative Example 5: The difference from Example 1 is that: after the aging process in step S5, anhydrous sodium carbonate is not added and the system is stirred rapidly at 800 rpm for 10 minutes to raise the pH of the system to 11. Instead, sodium hydroxide solution is added under stirring conditions to adjust the pH of the system to 11 and the system is stirred rapidly at 800 rpm for 10 minutes. The other conditions are the same as in Example 1.

[0035] Comparative Example 6: The difference from Example 1 is that: in step S5, an aqueous solution of poly(diallyldimethylammonium chloride) (concentration 20wt%, weight average molecular weight 300,000) is not added; the other conditions are the same as in Example 1, and the product of Comparative Example 6 is obtained.

[0036] Comparative Example 7: The difference from Example 1 is that in step S5, the washed precipitate is added to saturated calcium hydroxide clarification solution E and dispersed by stirring at 600 rpm for 10 min at 25°C. Instead of introducing carbon dioxide gas to lower the pH of the system to 11, the stirring is continued for 15 min. Then, 2000 g of deionized water is added and stirred for 5 min, followed by standing and removing the supernatant once. The system is then vacuum dried, pulverized, and sieved through a 200-mesh sieve. The remaining conditions are the same as in Example 1.

[0037] Performance testing: Sample preparation: All powder samples were dried in an oven at 105℃ for 2 hours and then cooled to room temperature in a desiccator for later use. For cement system tests, the same batch of P·O 42.5 ordinary Portland cement and the same batch of ISO standard sand were used, with deionized water as the mixing water. The admixture dosage was uniformly 2.0 wt% of the cementitious material (added in an equal amount to replace cement, i.e., keeping the total amount of cementitious material unchanged). The powder and cement were first dry-mixed for 2 minutes to ensure consistent dispersion. Unless otherwise specified, three parallel samples were prepared for each test item, and the results were taken as the arithmetic mean.

[0038] Specific surface area test (BET, according to GB / T 19587-2017): Take 0.5g of each powder sample and place it in the sample tube of the specific surface area analyzer. Degas at 200℃ for 4h under vacuum ≤10Pa. Perform nitrogen adsorption-desorption test in the 77K liquid nitrogen temperature range. Obtain the specific surface area S by BET fitting using data in the relative pressure range P / P0 = 0.05-0.30. BET (m) 2 / g).

[0039] Standard consistency water requirement and setting time of cement paste (according to GB / T 1346-2024): Take 500g of total cementitious materials, including 490g of cement and 10g of sample powder (2.0wt% equivalent to replace cement), dry mix for 2 minutes and then place in a mixing pot; the standard consistency water requirement is determined by the stepwise water addition method: add 150g of water initially and prepare the paste according to the standard mixing procedure, immediately pour into a mold and use a Vicat apparatus to measure the standard consistency needle sinking depth; if the sinking depth does not reach 6±1mm from the bottom plate, increase the water addition by 2g each time and repeat the paste preparation and measurement until the sinking depth reaches 6±1mm from the bottom plate, record the water addition at this time and convert it to the standard consistency water requirement P (wt%); the setting time is determined using cement paste with the same standard consistency water requirement as the sample, the test temperature is 20±1℃, and the relative humidity is ≥90%, with the water addition time as the starting point, and the initial setting time is defined as the time t corresponding to the Vicat initial setting needle sinking to 4±1mm from the bottom plate. i (min), final setting time is defined as the time t when the Vicat final setting needle leaves no obvious needle mark after being inserted into the specimen. f (min).

[0040] Cement mortar flowability (according to GB / T 2419-2005): The mortar mix ratio is as follows: total cementitious material 450g, including 441g cement, 9g sample powder (2.0wt% equivalent cement substitute), 1350g ISO standard sand, and 225g mixing water. The mortar is mixed in a mortar mixer according to the standard mixing procedure (low speed mixing with water added for 30s, high speed mixing for 30s, stopping for 90s to scrape the material, and then high speed mixing for 60s). The mortar is filled into a truncated cone mold in two layers. Each layer is tamped 20 times and leveled. After lifting the mold, it is dropped 25 times on a flowability table within 15s. The average value F (mm) of the mortar spreading diameter in two mutually perpendicular directions is measured.

[0041] Cement mortar compressive strength (according to GB / T 17671-2021): Using the same mortar mixing ratio and procedure as the cement mortar flowability test, prepare 40mm×40mm×160mm prism specimens, with no less than 6 specimens prepared for each sample; after molding, demold the specimens after standing for 24 hours at 20±1℃ and relative humidity ≥90%, and then cure them in water at 20±1℃ to the specified age; conduct flexural and compressive strength tests according to the standard, and test the compressive strength f of the two halves of the specimen after flexural testing, respectively, at 3d and 28d. c (MPa), and the average of 6 compressive strength values ​​at each age is taken as the result of the sample.

[0042] Natural drying shrinkage of building mortar (according to JGJ / T 70-2009): Using the same mortar mix proportion and mixing procedure as the cement mortar flowability test, 40mm×40mm×160mm prismatic shrinkage specimens were formed. The center diameter of the two end faces of the mold was Φ6.5mm, and shrinkage heads were installed, so that the shrinkage heads protruded 8±1mm from the end faces. After the specimens were vibrated and smoothed, they were placed in a pre-curing room at 20±5℃ for 4 hours, and then cured in the mold under standard curing conditions (20±2℃, relative humidity ≥90%) for 7 days before demolding. After demolding, the specimens were pre-cured in an environment of 20±2℃ and 60±5% relative humidity for 4 hours, and the initial length L0 (mm) was measured using a vertical mortar shrinkage meter. The specimen length L was measured again after 28 days. 28 (mm), calculate the natural drying shrinkage value ε at each age according to the standard formula. 28d The average value of the three specimens was used as the result, and the result was accurate to 10 × 10⁻⁶. -6 The test results are shown in Table 1.

[0043] Table 1 Performance Test Results

[0044] Data Analysis: Based on the data from the embodiments in Table 1, the modified attapulgite clay prepared by this invention exhibits a consistent synergistic performance pattern in cement system applications: through gradient preloading of calcium nitrate solutions A and B, introduction of poly(4-styrene sulfonate) graded by molecular weight and following a low-to-high sequence, and the synergistic effect of carbon dioxide gas-induced mineralization and anhydrous sodium carbonate densification processes, the overall specific surface area of ​​the powder decreases, the standard consistency water requirement decreases accordingly, the mortar fluidity increases accordingly, and the setting time is moderately advanced or maintained within a more compact range. On this basis, the material maintains high compressive strength in both the early and late stages, while natural drying shrinkage is significantly reduced. The underlying mechanism is speculated to be as follows: calcium ions are simultaneously enriched on the outer surface and within the pores, making it easier for the subsequent mineralization reaction involving sodium bicarbonate and carbon dioxide to nucleate directionally and grow continuously inside the pores or at the interface; low molecular weight poly(4-styrene sulfonate) segments preferentially penetrate into the pores and spatially locate calcium ion sites, while high molecular weight segments form a coating layer on the outer surface of the particles, thereby inhibiting the re-agglomeration of the particles and improving their dispersion stability in the cementing system; the inorganic functional layer formed by mineralization further plays a filling and interface anchoring role during the hardening process, reducing the connectivity of capillaries and the driving force of water loss and shrinkage, so that strength development and shrinkage inhibition can be achieved simultaneously in the same system.

[0045] Comparing the data from Example 1 and Comparative Example 1 in Table 1 reveals that if calcium nitrate solution B is no longer added for prolonged ion exchange in step S3, the specific surface area and standard consistency water consumption of the powder increase, the mortar fluidity decreases, the setting process is delayed, and both strength and shrinkage control show unfavorable changes. This is mainly because the lack of prolonged exchange makes it difficult for calcium ions to fully diffuse into the pores and intergranular spaces. The subsequent mineralization reaction triggered by sodium bicarbonate and carbon dioxide tends to occur more on the bulk phase or outer surface of the particles, resulting in insufficient continuity and anchoring of the functional layers inside the pores and at the interfaces. Therefore, it is difficult to simultaneously achieve the combined goals of reducing water demand, improving dispersibility, promoting hydration, and reducing shrinkage.

[0046] A comparison of the data from Example 1 and Comparative Example 2 shows that if poly(4-styrene sulfonate) solution C and D are premixed and added all at once, the water requirement for standard consistency increases, fluidity decreases, setting time is delayed, strength improvement is limited, and the shrinkage inhibition effect is weakened. The reason may be that adding it all at once weakens the advantage of low molecular weight segments preferentially penetrating the pores, while high molecular weight segments are more likely to undergo rapid charge coordination with calcium ions in the early stages, triggering local flocculation. This makes it difficult to effectively establish spatial division of labor between pore-localized nucleation and stable coating on the outer surface. This results in a more uneven distribution of mineralized products, an increased tendency for particle re-agglomeration, ultimately leading to increased water demand in the freshly mixed system and greater difficulty in achieving a dense hardened structure.

[0047] Comparing the data from Example 1 with Comparative Examples 3 and 4 reveals that when only high-molecular-weight or low-molecular-weight poly(4-styrenesulfonate) is used, it is difficult to simultaneously achieve the comprehensive balance shown in Example 1 in terms of setting time, fluidity, strength, and shrinkage properties. When only high-molecular-weight polymers are used, the outer surface coating of the particles is enhanced, but penetration into the pores is insufficient. This results in nucleation sites being concentrated on the outer layer of the particles, easily leading to localized organic enrichment and delayed hydration, resulting in a mismatch between fluidity and setting time, limited early strength, and excessive shrinkage. When only low-molecular-weight polymers are used, the intrapore positioning is improved, but the outer surface stabilization layer is insufficient. Particles are more prone to re-agglomeration in high-ionic-strength environments, leading to decreased dispersion stability and mineralization continuity. Therefore, introducing polymers according to molecular weight and following a sequence of infiltration followed by coating can produce unexpected synergistic effects, achieving simultaneous control of both the pore interior and outer surface scales, thus achieving a comprehensive performance improvement effect greater than the sum of its parts (1+1>2).

[0048] Comparative data from Example 1 and Comparative Example 5 show that if anhydrous sodium carbonate is not added after aging, and sodium hydroxide is used instead to adjust the pH of the system to 11, the standard consistency water consumption and specific surface area increase, the fluidity decreases, and the overall strength and shrinkage control deteriorate. The main reason is that anhydrous sodium carbonate not only provides carbonate ions to participate in the densification of the outer layer, but also promotes fine grain filling and defect sealing under a given mineralization sequence. Although sodium hydroxide can adjust the pH, it lacks the densification pathway involving carbonate ions, resulting in a more porous inorganic functional layer with higher pore connectivity, thereby weakening the microstructural basis upon which shrinkage and strength development depend.

[0049] A comparison of the data from Example 1 and Comparative Example 6 shows that if an aqueous solution of poly(diallyldimethylammonium chloride) is not added in step S5 for post-capture and stripping, the setting time is significantly prolonged, and the strength and shrinkage inhibition effects deteriorate simultaneously. A phenomenon occurs where the fluidity is relatively high, but the strength and shrinkage are unfavorable. The presumed reason is that while poly(sodium 4-styrenesulfonate) is beneficial for nucleation and localization during the mineralization stage, without the charge-pairing capture of poly(diallyldimethylammonium chloride) and the subsequent washing process, organic components are more likely to remain on the particle surface and in the pore channels. This hinders the migration of hydrated ions and crystal growth, and introduces higher porosity. Simultaneously, the lubricating and dispersing effect of the residual organic components on the freshly mixed system masks their adverse effects on the hardened structure.

[0050] Finally, comparing the data from Example 1 and Comparative Example 7 reveals that if carbon dioxide gas is not introduced into the saturated calcium hydroxide clarified solution E for secondary carbonization repair, the improvement in both strength and shrinkage is limited, and the specific surface area and water demand are difficult to reduce to the level of Example 1. This is mainly because secondary carbonization can re-seal and deposit defects and pore boundaries of the previously formed mineralized layer under a strongly alkaline environment, thereby enhancing the continuity of the functional layer and the interfacial anchoring effect. Without this step, the microcracks and pore defects of the mineralized layer are more difficult to repair effectively, resulting in a stronger driving force for water loss and shrinkage after hardening, insufficient densification, and thus limiting the simultaneous improvement of strength development and shrinkage reduction effect.

[0051] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing modified attapulgite clay, characterized in that, Includes the following steps: (1) Acid washing was performed on the attapulgite clay, and then it was washed until neutral to obtain neutral sediment. (2) Calcium nitrate solution A and calcium nitrate solution B are added sequentially to the neutral sediment for ion exchange to achieve calcium ion preloading, and the sediment is washed to obtain calcium ion preloaded sediment, wherein the calcium nitrate concentration in calcium nitrate solution A is higher than the calcium nitrate concentration in calcium nitrate solution B. (3) First, add poly(4-styrene sulfonate) solution C to the calcium ion preloaded sediment for reaction, and then add poly(4-styrene sulfonate) solution D for reaction to obtain a composite precursor slurry, wherein the weight-average molecular weight of poly(4-styrene sulfonate) in solution C is less than the weight-average molecular weight of poly(4-styrene sulfonate) in solution D. (4) Add calcium nitrate solution B and sodium bicarbonate to the composite precursor slurry, add sodium hydroxide alkaline solution to adjust the pH of the system to 9.8-10.2, and introduce carbon dioxide gas under stirring to reduce the pH of the system from 9.8-10.2 to 8.8-9.2 and mature it; after maturation, add anhydrous sodium carbonate to raise the pH of the system to 11.0-11.3; then add poly(diallyldimethylammonium chloride) aqueous solution to allow it to charge-pair with poly(4-styrene sulfonate) and form a precipitable polyelectrolyte complex, and wash the precipitate; (5) The washed sediment was dispersed in a saturated calcium hydroxide solution and carbon dioxide gas was introduced for secondary carbonization to reduce the pH of the system to 10.6-11.

5. The product after secondary carbonization was vacuum dried, crushed and sieved to obtain modified attapulgite clay.

2. The method for preparing modified attapulgite clay according to claim 1, characterized in that, In step (1), the average particle size of the attapulgite clay is 40-50 μm.

3. The method for preparing modified attapulgite clay according to claim 1, characterized in that, Based on 500 parts by mass of attapulgite clay, the preparation of calcium nitrate solution A is as follows: heat deionized water to 35-45℃, add 500-700 parts by mass of calcium nitrate tetrahydrate to dissolve, and then add deionized water to make up to 5000 parts by mass; the preparation of calcium nitrate solution B is as follows: dissolve 80-150 parts by mass of calcium nitrate tetrahydrate in deionized water and then add deionized water to make up to 5000 parts by mass.

4. The method for preparing modified attapulgite clay according to claim 1, characterized in that, In step (2), the ion exchange is carried out at 45-55℃, wherein the ion exchange reaction time of calcium nitrate solution A is 15-30 min and the ion exchange reaction time of calcium nitrate solution B is 90-180 min.

5. The method for preparing modified attapulgite clay according to claim 1, characterized in that, Based on 500 parts by mass of attapulgite clay, poly(4-styrene sulfonate) solution C is prepared by: heating deionized water to 35-45℃, dispersing and adding 4-10 parts by mass of poly(4-styrene sulfonate) and stirring until completely dissolved, then adding deionized water to bring the total to 4000 parts by mass. The weight-average molecular weight of the poly(4-styrene sulfonate) is 60000-80000. Poly(4-styrene sulfonate) solution D is prepared by: heating deionized water to 35-45℃, sprinkling in 0.5-2 parts by mass of poly(4-styrene sulfonate) and stirring for 2 hours, then adding deionized water to bring the total to 1000 parts by mass. The weight-average molecular weight of the poly(4-styrene sulfonate) is 900000-1100000.

6. The method for preparing modified attapulgite clay according to claim 1, characterized in that, In step (3), after adding poly(4-styrene sulfonate) solution C and reacting for 10-25 minutes, poly(4-styrene sulfonate) solution D is added at a uniform rate within 8-15 minutes, and then stirring is continued for 10-15 minutes.

7. The method for preparing modified attapulgite clay according to claim 1, characterized in that, Based on 500 parts by mass of attapulgite clay, in step (4), 800-1500 parts by mass of calcium nitrate solution B are added and stirred for 10 minutes, followed by 80-140 parts by mass of sodium bicarbonate and stirred for 10 minutes to completely dissolve it, and the system is kept at a constant temperature of 28-33℃.

8. The method for preparing modified attapulgite clay according to claim 1, characterized in that, In step (4), after stopping the introduction of carbon dioxide gas, continue stirring for 15-30 minutes for maturation. After maturation, add anhydrous sodium carbonate and stir for 10 minutes to raise the pH of the system to 11.0-11.

3.

9. The method for preparing modified attapulgite clay according to claim 1, characterized in that, Based on 500 parts by mass of attapulgite clay, in step (4), the concentration of the poly(diallyldimethylammonium chloride) aqueous solution is 20wt%, the weight-average molecular weight is 250,000-350,000, and the amount added is 20-60 parts by mass.

10. A modified attapulgite clay, characterized in that, It is obtained by the preparation method of the modified attapulgite clay according to any one of claims 1-9.

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